Espresso and other coffee and milk drinks are often prepared by mixing a powder in water. Traditionally, a milk froth is provided to the drink by steam frothing.
Mixing devices are known for speedier preparation of such beverages and other foods by mixing a powdered food component with a liquid, such as water. These devices typically feed the powdered component into the water, which is often pumped tangentially into the mixing chamber to create a whirlpool to mix the powder into the water. The mixture is then fed to a mixing mechanism—also called a whipping mechanism—, which is usually a rotating plate. The plate aerates the mixture and produces a froth. The frothed mixture is usually dispensed into a container for drinking.
U.S. Pat. No. 5,927,553, for example, discloses a mixing and dispensing apparatus with a cruciform frothing blade. Other shapes of frothing blades are also known. For instance, companies such as Rhea and Zanussi use whippers with an axially short disk with very steep sloped walls. Other whippers have rotors with independent ramps extending from a substantially flat plate. The known devices generally have their greatest efficiency for preparing a small group of products.
EP 1,116,464 of Bravilor discloses a mixing device comprising a motor 14 provided with a driving shaft 15 carrying a rotor 16,17. The rotor consists of a circular disc 16 provided with ribs 17. The ribs 17 are arranged on a circular end face of the disc 16 and extend from this end face in axial direction. How the ribs 17 exactly extend along the circular end face is neither described nor shown in EP 1,116,464. Only one rib is shown in side view. This rib appears to extend about diagonally over the circular end face of disc 16. The one rib 17 is at its radial outer ends tapered in the upstream direction. The product exit conduit 11 of EP 1,116,464 is arranged on the upstream side of the disc 16, radially beside the ribs.
WO 03/068039 of Nestle discloses a mixing device for aerating and frothing a product that includes a fluid component and at least one additional component. In this mixing device the rotor consists essentially of a tapering, preferably conically tapering, rotor body which is, on the tapering rotor surface provided with twisted, relatively small and accurately dimensioned grooves. The rotor housing has a corresponding tapered inner surface, but is a little larger so that there is a small, accurately defined gap between the housing and tapered rotor surface. Also at the downstream side of the rotor there is a small, accurately defined gap between the back surface of the rotor and the rotor housing. This accurate shape and accurate dimensions of the rotor and grooves provided in it as well as the accurate dimensions of the gap provide a very good frothing and aerating, which is highly desirable with instant coffee and milk drink machines. When the rotational speed of the rotor is sufficiently high the reliability of this mixing device on long term is also very good. However with rotational speeds of the rotor below 10,000 revolutions per minute (rpm) there occurs deposition of un-dissolved or partly dissolved components on the rotor and housing, which deposition results in clogging of the gaps and grooves. This clogging is in those kind of devices undesirable. Taking into account that the dimensions of gaps and grooves must meet very accurate requirements for optimal results, this known mixing device is relatively expensive in manufacturing and vulnerable for decrease in performance in case of clogging.
There is thus clearly a need for a mixing device, especially for instant drink machines, with a very good, if not improved, frothing and aerating effect, which is less, preferably not, vulnerable for decrease in performance as a consequence of clogging.
The object of the invention is to provide such an improved mixing device.